1. Field of the Invention
The present invention relates to a process of preparing yttrium-based superconductors, particularly to a process of preparing yttrium-based superconductors from a Y.sub.2 BaCuO.sub.5 compound reacting with liquid phase from a partially-melted YBa.sub.2 Cu.sub.3 O.sub.y compound.
2. Description of the Prior Art
Although very high critical current density is required for such practical applications as permanent magnets and power transmission, that of high Tc superconductors is very low and becomes much less in a magnetic field. This is known to be mainly due to weak-links between the superconducting grains.
To reduce these weak-links, melt processes, wherein a fused superconducting compound is solidified slowly, have been studied and attempted successfully. One of them is a quench and melt growth process (referred hereinafter as "QMG"), in which the stoichiometric compound of yttrium-based superconductors is heated rapidly to about 1400.degree. C. to form a homogeneous melt, and then cooled slowly to produce a superconductor. Partial melting is another melt process, in which a sintered stoichiometric superconductor is partially-melted to form a Y.sub.2 BaCuO.sub.5 phase and a liquid phase, and then is cooled slowly. The YBa.sub.2 Cu.sub.3 O.sub.y superconductor is produced by the peritectic reaction of the Y.sub.2 BaCuO.sub.5 phase with the liquid phase. The superconductors produced by these melt processes have large well-oriented grains with few voids, so that the weak-links are reduced drastically.
In these methods, however, liquid flowing by gravity cannot be avoidable. If the liquid flows out, the remaining compound suffers compositional change and sometimes cracks or distortion, all of which cause a lower critical current density. Even though rapid heating and/or instantaneous melting are attempted to prevent the liquid-flow, for example by placing the specimen directly in a pre-heated furnace, a thermal shock, in this case, cannot be avoidable, which is more harmful to the superconducting properties.